What does 69 mean i keep seeing it and people think its funny

The institute of international research is holding its annual convention. this year more than 500 scholars have been invited to hear research reports on many timely topics. approximately 250 big name scientists will be winging their way to washington, d.c. how can the underlined sentence be revised so that it fits the tone of the passage? a) approximately 250 people will be in washington, d.c. b) approximately 250 bigwigs will be flying to washington, d.c. c) approximately 250 science people will be going to washington, d.c. d) approximately 250 well-known scientists will be arriving in washington, d.c.